Syntheses, structures and properties of iron arsenic-based superconductors

Inspired by the layered feature of tetrahedrally-coordinated Fe 2Pn2 layers in the superconducting ZrCuAsSi type quaternary rare earth transition-metal oxypnictides, and using Zintl concept as an effective tool in rationalizing the structural properties and chemical bonding of these materials, the synthesis and properties of related superconducting iron pnictides AFe2As2 (A=Na, K, Rb, Cs) and AFeAs (A=Li, Na) were investigated. The alkali-metal-based KFe2As2, RbFe2As2, CsFe2As2 and LiFeAs, form a new family of intrinsic FeAs-based superconductors which do not need any additional doping to induce superconductivity. Superconductivity was induced in the spin density wave compound, SrFe2As2, by K or Cs substitution (chemical doping) of Sr. The complete pseudo-binary phase diagram of KFe2As2 and SrFe2As2 was obtained with a maximum of Tc at 37K at composition 40-50% K substitution in SrFe2As2 system. Spin density wave (SDW) states are also observed to occur at the Sr-rich side with decrease transition temperatures with respect to increasing K-content. Similar phase diagrams were investigated for all the AxAe1--x,Fe2As2 (A=Rb, Cs, Ac= Sr, Ba) systems, and nearly identical Tc maxima are observed. Furthermore in KxSr1--xFe2As2, the SDW transition is followed by a superconducting transition within a very narrow doping range. Extrapolation of the superconducting (SC) and SDW phase boundaries suggests the possible co-existence of SC and SDW as a quantum critical point. Weak ferromagnetism and loss of superconductivity is found for Co-doped K(Fe1--xCox)2As2. Unusual superconducting behavior is also observed for Na1--xFeAs system (x = 1--0.5). Mild oxidation reactions of NaFeAs in the ionic liquids resulted in a new metastable phase NaFe2As2 (ThCr 2Si2-type) that exhibits a broad superconducting transition with an onset at 25K. Additionally, a new ternary compound, CaFe 4As3, was synthesized in the Ca-Fe-As system. Its structure featured a FeAs network formed from condensation of Fe2As 2 layers that contains tetrahedrally-coordinated Fe atoms. The resulting 3D structure form nominal channels wherein Ca cations are located. Finally, the exploratory study on transition metal Fe pnictides resulted in several new compounds that crystallize in known structure types: (a) NbFeAs and ZnFeAs (Co2Si-type); (b) CuFeAs (Cu2Sb-type); and (c) TiFeAs (Fe2P-type). These compounds all feature a common building unit---FeAs 4 tetrahedra as in the Fe-pnictide superconductors. However, in these compounds, the edge-shared FeAs4 tetrahedra result in closer Fe-Fe interactions. More importantly, these non-superconducting intermetallic phases feature 3-dimensional structures.